A range finder based on laser technique has been successfully developed by Laser Tech Co., as disclosed in U.S. Pat. No. 404,779,552. The range finder can range 1 Km with precision within 1 m and includes a laser diode as a light source. There are two major drawbacks in the techniques employed in the prior arts. First, a method of fast charging and slow discharging is used to achieve the function of expanding the receiving time. Therefore, the time period (about 6.6 .mu.s) when the receiving circuit receives the respective echo light after the laser diode transmits the light, is expanded about 100 times. The time period is transformed into the distance by counting the samples (about 8 Mhz) at a low speed clock. The circuit in the finder is desired to use a current source with high accuracy to meet the requirement of fast charging and slow discharging which makes the device more unstable. Second, automatical threshold adjustment is used to optimize the sensitivity of the receiving circuit. However, the feedback sensitivity is too high to control.